Welcome!
I don't think your fish are dieing due to aggression, community fish won't generally injure each other unless breeding. I think your tank hasn't finished cycling yet, or maybe even someting more simple that that. What are your water parameters? Fine really doesn't help me, though I hope we can help you before it's too late for your fish. Next time if you're buying livebearers, buy 5 of each species to reduce aggression.
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If you have a big enough tank with enough hiding places, pH of around 7, you can keep virtually any fish together as long as all the fish are around the same size and these two groups of fish are avioded:
Serrasalmus
Tetradon(figure eights and dwarfs are the
exception).
I keep a successful community of fish in a 4 foot tank including the following families:
Cichlids, tetras, loaches, gouramis, barbs, rainbows, livebearers, killiefish, catfish, puffers.
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